The present invention is directed to clutch and brake assemblies and, more particularly, to clutch and brake assemblies having a clutch which is engaged during normal operation and brake which is disengaged during normal operation and further wherein the clutch is selectively disengageable and wherein the brake is automatically engageable the said clutch disengages.
Many common mechanical apparatus use clutches to selectively drive devices. In many of these mechanical apparatus, the driven device is driven during most of the time that the driving shaft is rotating. It is, therefore, advantageous, under these circumstances, to provide a clutch assembly having a clutch which is normally engaged and which is driven from engagement during the period of time that disengagement is desired. Many such clutches have been developed in the past and are well known in the art.
For many applications, however, these clutches must be coupled with a brake to prevent the inadvertent motion of the driven device when the clutch is disengaged. The undesired motion of the driven device is caused by friction between the drive shaft and the element, such as a pulley or a gear, interconnecting the driven element and the clutch mechanism. Alternatively, some motion may be caused by vibrations. Furthermore, if the driven element was driven prior to the disengagement of the clutch assembly, inertia may keep the driven element in motion for an undesired additional period of time.
A first example of an apparatus requiring a brake and clutch assembly is a print mechanism using a single drive shaft to advance a print head along a line of type and to separately advance a set of ink wheels along the line of type in a fixed relationship with the print head. In this apparatus, during normal operation, escapement of the print head is caused by a motor driving the drive shaft to rotate a predetermined angular amount. A clutch mechanism between the shaft and the print head, which is normally engaged, permits the rotation motion of the shaft to be transferred, by means of a belt or a wire tape, to the print head. A pulley rigidly interconnected with the shaft also drives a belt or wire tape interconnected with the color wheel assembly so as to advance the color wheel assembly along the line of type together with the print head.
The color wheel assembly, however, may consist of several color wheels each having a different color of ink. Rapid selection between the color wheels is desired so as to produce a high speed multi-colored output. Selection of a different color wheel is obtained by disengaging the above mentioned clutch and driving the drive shaft to rotate a predetermined angular amount so as to advance the color wheel assembly relative to the print head and, therefore, to align the print head with a different color wheel. Once the color wheel has been selected, the print head and the color wheel may be advanced along the line of type together by re-engagement of the clutch.
Clearly, in this apparatus, it is undesirable to have the print head move during the ink wheel selection process. Therefore, a brake is needed to prevent the motion of the print head during this period of time. To solve this problem, it had been proposed to provide a separate brake mechanism between the print head assembly and the frame of the printing apparatus. For example, a brake assembly may be mounted on the drive shaft and fixed against rotation by interconnection with the frame of the apparatus. Since the printing apparatus is electronically controlled, it is most convenient to use electromechanically actuated clutches and brakes. Therefore, the device would require two electromagnetic coils to actuate the brake and the clutch. These separate brake and clutch elements would use a large amount of valuable space and would generate a substantial amount of heat when actuated.
Print mechanisms are becoming smaller and smaller and greater demands, in terms of speed and in terms of flexibility, are being made on print mechanisms daily particularly due to the advent of microprocessors and large frame computers. It is, therefore, likely that additional selection features, such as the ink wheel selection feature described above, become common place in these printers. For example, selection of various fonts may occur in this manner. A clutch brake assembly, taking less space, consuming less power, and generating less heat than the above described separate clutch and brake elements is needed for this purpose.
A second apparatus requiring a normally engaged clutch and a normally disengaged brake is an air conditioning fan apparatus. When an air conditioner is started, it is advantageous to permit the Freon in the cooling circuit to begin flowing in the compressor to begin operation prior to drawing any air through the system. Therefore, during this period of time it is preferable that the fan not operate. Since this period of time represents a small fraction of the time during which the drive shaft will be rotating, a normally engaged clutch is used and the clutch is selectively disengaged during the start-up period of each cooling cycle.
During the start-up period, the fan may rotate at a slow angular speed due to vibrations and due to friction. It has been determined that this small amount of rotation is detrimental to the efficient operation of the air conditioning apparatus. Therefore, to avoid this problem, selectively closeable louvers are used to prevent the flow of air through the apparatus during the start-up period. Alternatively, a brake element may be used to inhibit this inadvertent rotation of the fan blades.
For this apparatus, it would be advantageous to have a single clutch and brake element which is easily manufactured and installed. It would be advantageous if such a clutch and brake element would consume a small amount of power during the start-up period of each air conditioning cycle and consume no power during the remainder of the cycle. It would further be advantageous if this element were compact.
What is needed, therefore, is a clutch and brake assembly having a clutch which is normally engaged and a brake which is normally disengaged which consumes no power and generates no heat during normal operation. Further, such a clutch and brake assembly should operate using a small amount of energy to selectively disengage the clutch and simultaneously engage the brake when required.